Multivariate linear regression (MLR)
The calibration was carried out using the least square method taking into consideration more than one explanatory variables Yi. Models were established during the MACPoll studies (see Table 2). Coefficients a, b, c, d and e represent calibration parameters extracted from the multi-linear regression; NO2, O3 and NO stand for the reference measurements. RH, T and H2O are respectively relative humidity, temperature and absolute humidity.